1. Field of the Invention
The invention relates to a digital image-capturing device, more particularly to a digital image-capturing device capable of maintaining optical axis alignment among components thereof.
2. Description of the Related Art
FIG. 1 illustrates a conventional digital optical instrument 10, such as a telescope, for viewing distant objects and for capturing images of the distant objects. The conventional digital optical instrument 10 includes a first conventional digital image-capturing device 12 mounted inside a casing 11 of the optical instrument 10.
The first conventional digital image-capturing device 12 includes an image sensor 13 (for instance, a CCD or a CMOS sensor) connected to a circuit board 14 and mounted collectively in the casing 11, an outer lens barrel 15 mounted in the casing 11 and spaced apart from the image sensor 13, an inner lens barrel 16 mounted threadedly in the outer lens barrel 15, and a lens module 17 mounted in the inner lens barrel 16. The device 12 has an optical axis (O).
The outer lens barrel 15 has a first central axis 151, and a first hole-confining surface 153 that surrounds the first central axis 151, that confines a first hole 152, and that is formed with an internal screw thread 154.
The inner lens barrel 16 has a second central axis 161, a second hole-confining surface 163 that surrounds the second central axis 161 and that confines a second hole 162, and an outer barrel surface 164 that is formed with an external screw thread 165 to engage threadedly the internal screw thread 154.
The lens module 17 includes a set of focusing lenses 173, 174, 175 and a light-filtering lens 176, all mounted in the second hole 162.
When an optical axis 177 of the lenses 173, 174, 175, 176 and an optical axis 131 of the image sensor 13 are aligned with the optical axis (O), the image quality of the images captured by the first conventional digital image-capturing device 12 is optimized, which is the goal manufacturers try to achieve.
Although the first conventional digital image-capturing device 12 has the ability to capture distant images, it has the following limitations:                1. Since the image sensor 13 and the outer lens barrel 15 are mounted separately in the casing 11, it is easy for the optical axis 131 of the image sensor 13 and the optical axis 177 of the lenses 173, 174, 175, 176 to deviate from the optical axis (O), thereby resulting in degradation of the quality of the images captured by the first conventional digital image-capturing device 12.        2. Since the image sensor 13 and the outer lens barrel 15 are mounted separately in the casing 11, and since there is no means for shielding between the image sensor 13 and the lens module 17, light leakage in the optical path between the image sensor 13 and the lens module 17 is likely to occur, which in turn degrades the quality of the images captured by the first conventional digital image-capturing device 12.        3. The inner lens barrel 16 is turned threadedly to move the inner lens barrel 16, as well as the lens module 17, closer to or away from the image sensor 13 along the second central axis 161 in order to perform focusing adjustments. However, due to gaps present between the internal screw thread 154 and the external screw thread 165, when the lens module 17 moves along with the inner lens barrel 16, the possibility for the optical axis 177 of the lenses 173, 174, 175, 176 to deviate from the optical axis (O) is high, thus resulting in image-quality degradation in the first conventional digital image-capturing device 12.        
As shown in FIG. 2, a second conventional digital image-capturing device 20 includes an outer lens barrel 21, an image sensor 22 connected to a circuit board 23 that is mounted to the outer lens barrel 21, an inner lens barrel 24 mounted threadedly in the outer lens barrel 21 and spaced apart from the image sensor 22, and a lens module 25 mounted in the inner lens barrel 24. The device 20 has an optical axis (O).
The second conventional digital image-capturing device 20 is built by mounting the various components thereof in the outer lens barrel 21. The outer lens barrel 21 is then mounted in the casing 11 of the optical instrument 10.
The second conventional digital image-capturing device 20 is capable of overcoming the aforementioned first and second limitations of the first conventional digital image-capturing device 12. However, since the outer lens barrel 21 and the inner lens barrel 24 are interconnected threadedly through an internal screw thread 211 and an external screw thread 241, implying that there are gaps present between the internal screw thread 211 and the external screw thread 241, during focusing adjustments, the optical axis 251 of the lens module 25 is likely to deviate from the optical axis (O), thus resulting in image-quality degradation in the second conventional digital image-capturing device 20.